Single coil, direct current permanent magnet brushless motor with voltage boost

ABSTRACT

A single coil, direct current permanent magnet brushless motor including a stator including six alternately-wound coils connected into a single coil having first and second ends, the oppositely-wound coils forming stator poles, and six magnets of alternating polarity coupled to a rotor and rotatably journaled in the stator. A sensor, such as a dual output Hall sensor, is used for sensing rotation of the rotor. A drive circuit, such as an H-bridge circuit, is coupled to the first and second ends of the single coil to drive the motor. The H-bridge circuit includes two high-side switches for alternately receiving signals from the Hall sensor, and two low-side switches alternately receiving signals from the Hall sensor. A high-side switching signal can be controlled by an inverted low-side switching signal. A voltage boost circuit is also provided, having capacitors to provide a boosted voltage to alternately turn on the high-side switches of the H-bridge. The capacitors can be charged by an unregulated bus voltage.

TECHNICAL FIELD

[0001] This invention relates generally to direct current electricmotors. More particularly, this invention relates to a single coil,direct current permanent magnet brushless motor with a voltage boostcircuit.

BACKGROUND

[0002] Permanent magnet brushless electric motors are desirable forefficiency. Brushless motors are typically more efficient and quieterthan induction motors because brushless motor designs avoid lossesrelated to the “induction” process. However, the costs associated withthe manufacture of brushless motors are usually greater than inductionmotors. For example, brushless motors can be more expensive thaninduction motors because of the control circuitry necessary to drive thebrushless motors. Therefore, until recently, brushless motors havetypically been used in larger, expensive equipment such as washingmachines and high-efficiency furnaces and in medical and militaryapplications, where cost is less of a factor.

[0003] Increased concerns for efficiency and stricter governmentregulations are requiring more efficient electric motors. Single-phasebrushless motors are known. See, for example, U.S. Pat. Nos. 4,379,984,4,535,275 and 5,859,519, and S. Bentouati et al., Permanent MagnetBrushless DC Motors For Consumer Products (last visited Dec. 8, 2002),located at URL mag-net.ee.umist.ac.uk/reports/P11/p11.html.

[0004] Although different brushless motors can vary in configuration,all brushless motors run on direct current and include circuitry tosequentially switch the direct current into one or more stator coils. Inaddition, most brushless motors include a plurality of permanent magnetsattached to a rotor.

[0005] Brushless motors typically have a different number of statorpoles versus rotor poles. For example, a majority of brushless motormanufacturers use a three phase drive circuit including three rotationsensors and six transistors to switch the direct current. Current flowsthrough two of the three coils or phases at any one time. Therefore, athree phase motor with three coils only utilizes approximatelytwo-thirds of the copper windings at one time. Such a configuration canprovide a smooth drive and good starting torque, but is complicated interms of the number of components and the expense of the components.Other similarly designed motors including different pairings of statorpoles versus rotor poles (e.g., 6-8, 12-8, 4-6, 6-2) are also complexand expensive.

[0006] In particular, the circuitry used to drive a brushless motor canbe complex and expensive. For example, some drive circuits for brushlessmotors require a voltage boost, or discrete isolated voltage sources.This can be accomplished, for example, using a transformer. However,transformers are both bulky and expensive. Voltage doublers can also beused, but they typically require large and expensive capacitors togenerate the needed voltages with sufficient current capability. Othercircuitry, such as charge pumps with a dedicated oscillator, diodes, andcapacitors, has also been used.

[0007] One application in which the above-described voltage boostcircuits have been used is in drive circuits for brushless motorsincluding a main semi-conductor switch (e.g., mosfets, transistors,SCRs, Triacs, etc.) that “is above the load.” This is generally the casein a drive circuit in which a full-bridge or half-bridge is used todrive the motor. Although the drive circuits noted above may be used ina drive circuit for a brushless motor with main switches that are abovethe load, such circuits can be inefficient, complex, andcost-prohibitive.

[0008] Accordingly, it is desirable to provide a brushless motor that isefficient and can be manufactured in a cost-effective manner.

SUMMARY

[0009] This invention relates generally to direct current electricmotors. More particularly, this invention relates to a single coil,direct current permanent magnet brushless motor with a voltage boostcircuit.

[0010] According to one aspect, the invention relates generally to asingle coil, direct current permanent magnet brushless motor, includingan internal rotor with six alternate polarity magnets rotatablyjournaled in the motor, and an external stator with six salient polesincluding six alternately wound coils coupled to form a single coil withtwo free ends. The motor can also include a commutated H-bridge having avoltage boost circuit with capacitors providing a boosted voltage toalternately turn on high-side switches of the H-bridge, wherein thecapacitors are charged by a switching current flowing through low-sideswitches.

[0011] In another aspect, the motor can also be configured to be poweredby either alternating current or direct current. For example, the motorcan have an alternating current conversion circuit including a bridgerectifier and smoothing capacitor coupled to a source of alternatingcurrent, the conversion circuit converting the alternating current toprovide direct current to power the motor.

[0012] In yet another aspect, a means for providing locked rotorprotection can include a Hall sensor configured to turn off the twohigh-side switches and two low-side switches of the H-bridge for aperiod of time when the Hall sensor detects a locked rotor condition.

[0013] The above summary is not intended to describe each disclosedembodiment or every implementation of the present invention. Figures andthe detailed description that follow more particularly exemplifyembodiments of the invention. While certain embodiments will beillustrated and described, the invention is not limited to use in suchembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Aspects of the invention may be more completely understood inconsideration of the following detailed description of variousembodiments of the invention in connection with the accompanyingdrawings, in which:

[0015]FIG. 1 is a partial cutaway view of an example single coil, directcurrent permanent magnet brushless electric motor;

[0016]FIG. 2 is a schematic view of six coils coupled to form a singlecoil with two free ends;

[0017]FIG. 3 is a perspective view of another example single coil,direct current permanent magnet brushless motor including a schematic ofan example commutation circuit including voltage boost;

[0018]FIG. 4 is a perspective view of another example single coil,direct current permanent magnet brushless motor including a schematic ofan example commutation circuit and an alternating current conversioncircuit; and

[0019]FIG. 5 is a perspective view of another example single coil,direct current permanent magnet brushless motor including a schematic ofan example commutation circuit having a microcontroller.

[0020] While the invention is amenable to various modifications andalternative forms, specifics thereof have been shown by way of exampleand will be described in detail. It should be understood, however, thatthe intention is not to limit the invention to the particularembodiments described. On the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention.

DETAILED DESCRIPTION

[0021] This invention relates generally to direct current electricmotors. More particularly, this invention relates to a single coil,direct current permanent magnet brushless motor with a voltage boostcircuit. While the present invention is not so limited, an appreciationof the various aspects of the invention will be gained through adiscussion of the examples provided below.

[0022] Generally, the present disclosure relates to a single coil,direct current permanent magnet brushless motor including a rotor withalternate-polarity magnets rotatably journaled in the motor and a statorwith a like number of stator poles including wound coils connected intoa single coil with two ends. Preferably, the motor includes at leastfour magnets and a like number of stator poles. More preferably, themotor includes six magnets and six stator poles. In addition, the motorincludes a commutated H-bridge coupled to the two ends of the singlecoil to drive the motor.

[0023] Referring now to FIG. 1, one embodiment of a single coil, directcurrent permanent magnet brushless motor 100 is shown. Generally, themotor 100 includes a stationary stator 170 and a rotatable rotor 160.Preferably, an air gap 180 formed between the stator 170 and the rotor160 is concentrically uniform, irrespective of any reluctance notchesformed in the stator.

[0024] The stator 170 includes a plurality of stator poles 110individually wound and connected to form a single coil 105 with two freeends 120 and 130 (see FIG. 2). The single coil 105 can be formed using avariety of techniques such as, for example, a bifilar winding. Eachstator pole 110 is formed by winding a coil in a given direction. Eachalternating pole 110 is wound in an opposite direction and connected tothe next pole to form an alternating series of north and south statorpoles.

[0025] In addition, the rotor 160 of the motor 100 includes a pluralityof rotor poles 140, formed by permanent magnets coupled to the rotor160. Each alternating rotor pole 140 is of a different polarity to forman alternating series of north and south rotor poles. The illustratedrotor 160 is an internal rotor, although external or flat-type rotorscan also be used.

[0026] Preferably, at least four alternating stator poles and fourassociated rotor poles are provided. Preferably, the brushless motorincludes the same number of stator and rotor poles. Most preferably, andas illustrated, the motor 100 includes six stator poles and a likenumber of rotor poles.

[0027] To operate the motor 100, free ends 120 and 130 of the singlecoil 105 are connected to a source of electric power. Specifically, thefree end 120 is connected to an electric source of positive potential,and free end 130 is connected to an electrical source of negativepotential. In this configuration, electrical current flows through thesingle coil 105 in a forward direction, for example, from free end 120to free end 130. As the current flows through the single coil 105, thestator poles 110 act as electromagnets of alternating north or southpolarity, depending on which direction each stator pole 110 is wound.

[0028] The rotor poles 140 are attracted to each respective adjacentoppositely-charged stator pole 110, causing the rotor 160 to turn. Asthe current flowing through the single coil 105 is alternately switchedbetween the forward and a reverse direction, each stator pole 110changes polarity to attract an oppositely-charged rotor pole 140,causing the rotor 160 to continue spinning. One pulse (i.e. the changein the direction of the current through the single coil 105) is requiredfor each pole to cause the rotor to complete a full revolution of 360degrees. For the illustrated six-pole motor, six pulses are required tocause the rotor 160 to complete one full 360-degree revolution. As therotor 160 spins, torque is transferred to a shaft 150 that is coupled tothe rotor 160 of the motor 100.

[0029] A sensor (not shown in FIG. 1) that can be fixed on the stator,in close proximity to the permanent magnets on the rotor, is able todetermine the polarity of the magnet positioned in front of it. Thesensor is thereby used to provide feedback as to the angular position ofthe rotor 160 relative to the stator 170 to control the direction of thecurrent (forward or reverse) applied to the first and second ends 120and 130 of the single coil 105, thereby providing the switchingnecessary to cause the rotor 160 to spin.

[0030] Multiple speeds for the motor 100 can be accomplished, forexample, with pulse circuits including pulse width modulation (PWM),phase control, or multiple windings, or by switching in a currentlimiting capacitor in an alternating current line, if the motor isdriven by rectified alternating current as described in U.S. Pat. No.4,929,871 to Gerfast.

[0031] Referring now to FIGS. 3 and 4, example single coil, directcurrent permanent magnet brushless motors 200 and 300 are shownincluding example drive circuits 210 and 310. The motor 200 shown inFIG. 3 is powered using a direct current (DC) source, while the motor300 shown in FIG. 4 is powered using an alternating current (AC) source.The drive circuits 210 and 310 can commutate current through the singlecoil 105 to cause the motors 200 and 300 to spin, as described above.

[0032] Referring to FIG. 3, the drive circuit 210 includes semiconductorswitches 218, 219, 225 and 226. In a preferred embodiment, N-channelmosfets with a 60 to 600 volt rating and about 35 nanosecond switchingare used. However, other semiconductor switches such as other mosfets(e.g., P-channel or PNP), SCRs, Triacs, or other transistors, forexample, can also be used.

[0033] The circuit 210 also includes inverters 221 and 222 andcapacitors 223 and 224, described further below.

[0034] The switches 218 and 219 function as high-side switches, and theswitches 225 and 226 function as low-side switches. The drains of thetwo high-side switches 218 and 219 are connected to the bus voltage,while the sources of the two low-side switches 225 and 226 are connectedto ground. The source of the high-side switch 218 and the drain of thelow-side switch 226 are connected to the second end 130 of the singlecoil 105, while the source of the high-side switch 219 and the drain ofthe low-side switch 225 are connected to the first end 120 of the singlecoil 105.

[0035] The drive circuit 210 drives the motor 200 as follows. Generally,the driver circuit 210 switches the direction of the current flowingthrough the single coil 105. When high-side switch 218 and oppositelow-side switch 225 are turned on, current flows in a first or “forward”direction through the coil 105. When switches 218 and 225 are turnedoff, and high-side switch 219 and low-side switch 226 are turned on,current flows in a second or “reverse” direction through the coil 105.As noted above, alternating the direction of the flow of current throughthe coil 105 causes the rotor 160 to spin, and torque is therebytransferred to the shaft 150.

[0036] To initiate the change in the state of the switches, a sensor 220is used to measure the angular position of the rotor poles 110 withrespect to the stator poles 120. In a preferred embodiment, a singlesensor is used, regardless of the number of poles in the motor. Alsopreferred is a dual output Hall sensor that is mounted to the stator 170adjacent the rotor 160. As the rotor 160 spins, the sensor 220 measuresthe change in polarity as oppositely-magnetized rotor poles 140 pass bythe sensor. As the rotor pole 140 (and its associated polarity)positioned in front of the sensor 220 changes, the sensor 220 measuresthe change and provides the commutating signal in order to change thedirection of the current flowing through the coil 105.

[0037] In alternative embodiments, sensors other than a dual output Hallsensor can be used. For example, a single output Hall sensor can beused, as well as an optical sensor. In addition, multiple sensors can beprovided. The sensors can also perform functions other than measuringthe angular position of the rotor such as, for example, measuring whenthe rotor has stopped spinning to provide locked rotor protection, asdescribed further below.

[0038] More specifically, the circuit 210 can be used to commutate thecurrent flowing through the coil 105 as follows. When an output 220 a ofthe sensor 220 is positive, an output 220 b is always the opposite ofoutput 220 a (i.e. negative). When the polarity of the magnet positionedin front of the sensor 220 causes the sensor 220 to provide a positivesignal on output 220 a, the switch 225 is immediately turned on. Thesame signal from the output 220 a of the sensor 220 is also provided atthe inverter 221, which inverts the signal, providing a negative signalto the switch 219, turning it off. The output 220 b of the sensor 220 isopposite of that of 220 a, therefore turning off switch 226 whileturning on switch 218. The result is that direct current flows throughswitches 218 and 225 to ground, thereby producing a torque in the coilthat swings in an opposite polarity to that of the magnet in front ofthe sensor 220. The torque is transferred to the rotor, causing therotor to spin, and thereby causing the sensor 220 to transition to asecond state as another magnet of opposite polarity swings into positionin front of the sensor. This causes the sensor to change the outputs 220a and 220 b, thereby turning switches 218 and 225 off and 219 and 226on, causing the direct current to flow in the opposite direction throughthe coil.

[0039] The high-side switch 218 requires a gate voltage higher than itssource voltage to turn on. If the voltage at the end of coil 105 that isconnected to switch 226 is lower than the voltage at point 269 thencapacitor 224 will be charged to the voltage level at 269. When switch226 and inverter 222 are turned off, capacitor 224 will provide voltageto the gate of high-side switch 218 and switch 218 will turn on. Whileswitch 218 remains on, the voltage on capacitor 224 will be higher thanthe bus voltage. Accordingly, high-side switch 219 will be turned onwith the voltage from capacitor 223 when switch 225 and inverter 221 areoff.

[0040] The illustrated switching scheme is therefore advantageous inthat an unregulated voltage source can be used to charge the voltageboost capacitors. In this “unregulated” configuration, the voltageacross the capacitors 223 and 224 remains at a desired value withoutrequiring a voltage regulator or separate isolated voltage source.

[0041] In FIG. 3, the voltage at point 269 is the same as the applied DCvoltage. In FIG. 4, the voltage at point 269 is produced by a voltagedivider. The voltage divider is either resistors 270 or 272 in serieswith 273. The drive circuit 310 shown in FIG. 4 is similar to drivecircuit 210 described above, except for an alternating currentconversion circuit 315. The circuit 315 accepts at inputs 332 and 334current from an AC source. A bridge rectifier 313 (including fourdiodes) and a smoothing capacitor 323 are used to convert the AC intoDC, which powers the remainder of the driver circuit 310. In thismanner, an AC source is used to drive the motor 300.

[0042] In addition to requiring drive circuits to function, electricmotors may require locked rotor protection to increate reliability. Thisprotection can take the form of thermally operated switches or relaysthat are sufficient to protect induction motors that heat up slowly withthe rotor locks up. An electronically driven motor such as a brushlessmotor uses transistors that heat up rapidly, therefore other methods ofsensing rotor lockup may be required.

[0043] In the illustrated embodiment, locked rotor protection can beprovided by the sensor 220. The preferred Hall sensor is a dual outputHall sensor that is configured to drive two inductive coils, with anadded feature to detect a stalled condition. In the illustratedembodiment, the sensor 220 is modified by the addition of two resistors270 and 272 to supply current to the sensor. When the rotor is locked upor stalled, the sensor 220 detects an absence of magnetic change andthis condition is reflected at the resistors 270 and 272, with thesensor shutting off current to all four switches 218, 219, 225, and 226for a period of time.

[0044] In this configuration, locked rotor protection is achieved withminimum parts and at a low cost. Other methods can also be used toprovide locked rotor protection, such as by using a sensor resistor andan SCR, with the sensing resistor positioned in the main line to providea turn-off when current increases rapidly during locked rotorconditions. In such an arrangement, the gate of the SCR is provided witha “hold-off” capacitor and diode to prevent false turn-offs.

[0045] A brushless electric motor configured as disclosed herein hasseveral advantages. For example, the preferred six-pole brushless motordisclosed herein includes only two free ends, which can be driven with adrive circuit that is simple in terms of the number of components. Forexample, only the four transistors formed into a bridge circuit areneeded. Other single phase motor designs, including 4, 8, or 10 poles,likewise include only two free ends and are therefore advantageous. Inaddition, the brushless motors disclosed herein are cost-effective formanufacture, and are as efficient or more efficient than other brushlesselectric motors, since approximately 100 percent of the copper windingsare utilized at a given time. Further, the drive circuits for the motorsare robust and can provide efficient locked rotor protection usingminimal additional components.

[0046] Various modifications can be made to the motor and circuits shownand described herein. For example, as shown in FIG. 5, example circuit410 includes a microcontroller 412 and drivers 415 and 416 that arecoupled to the Hall sensor 220 output and are used to commutate theH-bridge circuit.

[0047] In other embodiments, various forms of digital signal processingcan be used to enhance commutation of the motor. Other modifications tothe motor and circuitry are also possible, such as commutation without aHall sensor.

[0048] The above specification, examples and data provide a completedescription of the manufacture and use of various aspects of theinvention. Since many embodiments of the invention can be made withoutdeparting from the spirit and scope of the invention, the inventionresides in the claims hereinafter appended.

What is claimed is:
 1. A single coil, direct current permanent magnetbrushless motor, comprising: a rotor with at least four alternatepolarity magnets rotatably journaled in the motor; a stator with a likenumber of salient poles, each including alternately wound coils coupledto form a single coil with two free ends; and a commutated H-bridgeincluding a voltage boost circuit having capacitors providing a boostedvoltage to alternately turn on high-side switches of the H-bridge,wherein the capacitors are charged by a low-side switching signalflowing through low-side switches.
 2. The motor of claim 1, wherein ahigh-side switching signal is controlled by inverting the low-sideswitching signal.
 3. The motor of claim 1, wherein the capacitors arecharged by an unregulated bus voltage.
 4. The motor of claim 1, whereinthe motor includes six alternate-polarity magnets and six coilsconnected into the single coil.
 5. The motor of claim 1, wherein theH-bridge further includes an alternating current conversion circuitincluding a bridge rectifier and smoothing capacitor coupled to a sourceof alternating current, the conversion circuit converting thealternating current to provide direct current to power the motor.
 6. Themotor of claim 1, wherein the motor is configured to be powered byeither alternating current or direct current.
 7. The motor of claim 1,further comprising a microcontroller to commutate the H-bridge.
 8. Themotor of claim 1, wherein a uniform concentric air gap is definedbetween the stator and the rotor.
 9. The motor of claim 1, furthercomprising means for providing locked rotor protection.
 10. The motor ofclaim 9, wherein the means for providing locked rotor protectionincludes a Hall sensor configured to turn off the two high-side switchesand two low-side switches of the H-bridge for a period of time when theHall sensor detects a locked rotor condition.
 11. The motor of claim 1,wherein the rotor is internal with respect to the stator.
 12. The motorof claim 1, wherein the motor requires a number of pulses equal to thenumber of stator coils to cause the rotor to complete a full revolution.13. A single coil, direct current permanent magnet brushless motor,comprising: an internal rotor with six alternate polarity magnetsrotatably journaled in the motor; an external stator with six salientpoles including six alternately wound coils coupled to form a singlecoil with two free ends; a means for providing locked rotor protection;and a commutated H-bridge including a voltage boost circuit havingcapacitors to provide a boosted voltage to alternately turn on high-sideswitches of the H-bridge, wherein the capacitors are charged by anunregulated bus voltage.
 14. The motor of claim 13, wherein the H-bridgefurther includes an alternating current conversion circuit including abridge rectifier and smoothing capacitor coupled to a source ofalternating current, the conversion circuit converting the alternatingcurrent to provide direct current to power the motor.
 15. The motor ofclaim 13, wherein the motor is configured to be powered by eitheralternating current or direct current.
 16. The motor of claim 13,further comprising a microcontroller to commutate the H-bridge.
 17. Themotor of claim 13, wherein a uniform concentric air gap is definedbetween the stator and the rotor.
 18. The motor of claim 13, wherein themeans for providing locked rotor protection includes a Hall sensorconfigured to turn off the two high-side switches and two low-sideswitches of the H-bridge for a period of time when the Hall sensordetects a locked rotor condition.
 19. The motor of claim 13, wherein themotor requires a number of pulses equal to the number of stator coils tocause the rotor to complete a full revolution.
 20. A method ofcommutating a single coil, direct current permanent magnet brushlessmotor including a rotor with at least four magnets rotatably journaledin the motor and a stator with a like number of salient poles eachhaving alternately wound coils coupled to form a single coil with twofree ends, and a commutated H-bridge including a voltage boost circuithaving capacitors providing a boosted voltage to alternately turn onhigh-side switches of the H-bridge, the method comprising: charging thecapacitors by a switching current flowing through low-side switches ofthe H-bridge; controlling the high-side switches of the H-bridge usingan inverted low-side switching signal from the low-side switches of theH-bridge; and turning on the high-side switches of the H-bridge usingthe charge stored in the capacitors.
 21. The method of claim 20, furthercomprising providing locked rotor protection.
 22. The method of claim20, further comprising providing an alternating current conversioncircuit to allow the motor to be powered by alternating current.